Application of protective coating to industrial and commercial products is essential. The utilization of electrostatic powder spray guns for applying these coatings is also well known in the art. Protective coating or powder Coating is dry paint. The finely ground powder particles made from pigment, resin and other additives are given electrostatic charge and sprayed onto a metal part. The parts to be coated are grounded neutral so that the charged particles projected at them adhere to the parts and are held there until melted and fused into a smooth coating in the curing oven.
The largest volume of powder coating is applied by electrostatic spray using equipment especially designed for powders. Other methods of application are fluidized powder bed dipping, electrostatic fluidized bed and flocking. This electrostatic application of powder is made possible because powders, being generally non-conductive, will retain an electrostatic charge and cling to a grounded substrate until melting in the cure oven. The charge may be positive or negative, but negative is the most efficient for most materials and is most widely used.
A typical electrostatic spray application system has a container for powder storage, delivery system and a charging system. The basic pieces of a typical equipment that make up a spray unit are a feed hopper or powder box container, a powder spray gun connected to a powder suction pump, an electrostatic power source (cascade), the controls. These components are connected by hoses and cables and all the necessary regulators and fittings to complete the package. The feed hopper is designed the same as a fluidized tank.
In order to apply a powder coating over a specific surface in a uniform pattern a charge is supplied to the material. The powder must be charged to adhere. In a corona gun the powder particles are passed through a charging field by the delivery system and attracted to the object to be coated which is earthed to the ground.
Another method of spray application is the tribo-charging gun. In a tribo-charging gun, the powder particles are charged by frictional contact with the materials inside the gun body.
The function of the powder spray gun is to shape and direct the flow of the powder, help to control the spray pattern shape and size, and impart an electrostatic charge to the powder.
Powder output and charge, and therefore powder deposition efficiency are controlled by the delivery system and the charging system. The delivery system sucks and transports the powder from the container to the gun tip and out of the spray gun. The charging system charges the powder material so that it will be attracted to the part and stick to the part surface until it is cured. The pneumatic controls and the voltage control are very important elements for achieving good transfer efficiency.
Powder is sprayed from the tip of the gun of the powder spray equipment. This powder is charged. This charged powder is sprayed onto the work-piece which needs to be powder coated. This charged powder which is sprayed adheres to the work-piece due to the electrostatic charging. The over sprayed powder is then collected in a chamber and recovery system such as powder spray cum recovery booth. The ratio of the amount powder adhering to the work-piece to the amount of powder sprayed out of the powder spray equipment is called transfer efficiency.
Moisture and other factors make the powder coagulated and lumpy in spite of the best prior sieving. Powder is hygroscopic and moisture can be picked up by the powder while lying in the powder container or while it is being transported from the recovery units back to the powder container for re-use. This coagulation of the moist powder is a big source of an un-even powder coat, poor transfer efficiency, poor flow and poor finish. Powder is collected in the recovery of the powder spray cum recovery booths. At this stage powder tends to contaminate with dust and other particles. This powder has to then be transported back to the master powder container either by pneumatic or mechanical means. This also sometimes leads to contamination of the recovered powder. The powder is also subject to exposure to moisture and being hygroscopic it often does pick up this moisture and get lumped. Since, the powder is transported in boxes and during transportation it gets compacted in the box itself, the powder needs to be sieved prior to use.
Normally this sieving is done by means of vibrating sieves. The vibrating methods generally followed are pneumatic, electric, ultra-sonic, mechanical or any other means. After this sieving the powder again needs to be transported for spraying to the master powder container to which the powder pumps and spray guns are connected. This transportation is once again a source of contamination and coagulation. Thus, you can still have contaminated and lumped powder prior to spraying, in spite of the costly and time consuming sieving operations.
The standard method of powder sieving is to pass the powder through a sieve device and the sieving takes place due to a vibratory or gyratory or rotary motion of the sieves using known means such as pneumatic, electric, mechanical or ultrasonic methods. These sieve machines may be placed under the recovery system or may be separately placed outside of the powder recovery system. They can be quite expensive, bulky and complex in operation. They need a separate area for placement which has to be accommodated in the plant design. The sieve machines after a while have to be cleaned periodically either manually or by designing some automated method and this is an expensive and cumbersome process.
In case of the sieve machines integrated with the recovery units or placed under the recovery zone or placed separately besides the recovery system, the powder after sieving has to be collected or transported into the master container to which the powder output device is connected. The sieving and transportation of the sieved powder to the container for spraying involves an additional procedure and is quite cumbersome.
In case of Batch sieve or stand alone sieve machines, the powder after collection has to be manually taken to the sieving machines and then after sieving the powder has to be recollected and transported to the master container. The transportation of the powder to the sieve and collection and re-transportation to the master container for spraying involves costly and labour intensive operations such as pumping devices manual transportation, human labour, possible spillage and contamination.
In spite of the best sieving of the powder prior to transportation to the master spray container for further spraying, impurities can creep in during this process. Powder in the master spray container often still gets contaminated or even coagulated due to various reasons such as absorption of moisture as it is continuously fluidized for long periods without being consumed especially since the powder is hygroscopic in nature. Another point of contamination is that the powder remaining stagnant overnight or change of shift. In such case sieved powder is subsequently contaminated gets sprayed in the contaminated form.
Further, in conventional existing powder coating systems, the fresh and recollected powder has to be transported (either manually or automatically) to bulky and expensive sieving machines. It was conventional wisdom to have large and bulky sieves to handle the quantity of powder to be sprayed. The sieved powder needs to be re-transported to the main spray powder container or box for further spraying from the gun.
It is therefore required to formulate a mechanism to overcome the problems associated with the prior art machines.